Calving along the edge of world's fastest-moving glacier near Jacobshavn, Greenland. Studies indicate the ice sheet is shrinking about 3 feet or more a year.
(Kurt Suplee - The Washington Post) |
Greenland Is Skating on Thinner Ice
1st Precise Measurements Show Melting Accounts for 7% of World's Sea-Level Rise
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The massive Greenland Ice Sheet--which contains nearly 10 percent of all the frozen water on Earth--is melting at a rate of about 12 cubic miles per year, accounting for 7 percent of sea-level rise worldwide.
That estimate, the most comprehensive to date and the first ever based on precision measurements, is the conclusion of new laser surveys by scientists at NASA's Goddard Space Flight Center in Greenbelt who have measured the height of the ice sheet from aircraft since 1993.
"It's amazingly stable at the highest elevations" (above 6,500 feet), said William Krabill, who with colleagues reports the findings in today's issue of the journal Science. A second paper in Science by another Goddard group supports that conclusion based on Global Positioning System measurements that show no net loss in the high parts of the central ice sheet over the past few decades.
"However, around the margins it's undergoing fairly significant thinning," Krabill said, shrinking 3 feet or more per year where the ice flows into the ocean.
The exact reason for the mass loss is unknown, though a likely suspect is an increase in what glaciologists call "creep rate," the speed at which an ice sheet moves. "If you get melting on the surface," Krabill said, "it flows down through the crevasses and ends up lubricating the bed," where the ice meets the underlying rock. That, in theory, makes it easier for the whole mass to move.
Equally uncertain is whether the melting is highly abnormal or has increased in recent years, perhaps as a consequence of global warming. That is a matter of urgent concern to climate scientists and to social planners along with the 50 percent of humanity that lives in coastal areas. Nations ranging from Holland to Bangladesh could lose a substantial portion of their land to rising oceans, and seaside developments are vulnerable to changing water levels.
"NASA's work on Greenland ice cap mass balance is a critical part of the global change picture because of the enormous potential impact on sea level," said Michael T. Ledbetter, director of the National Science Foundation's Arctic System Science Program.
Understanding overall and localized changes in the Greenland Ice Sheet is essential to projecting the near-term future of the world's ice. "We've been waiting to get this kind of comprehensive measurement for decades," said glacier expert Robert Bindschadler of GSFC, president of the International Glaciological Society.
The ice studies, conducted by aiming an airborne laser altimeter down and measuring the surface elevation shifts, represent only five years of data. "That's an awfully short amount of time in glaciological sense," Krabill said. "It's a sort of a snapshot."
In 1993 and 1994, the researchers took readings along various flight lines around Greenland. Then in 1998 and 1999, the group flew over the same routes to determine what changes had occurred. The new report combines those data, and attempts for the first time to calculate the ice loss by factoring in the warming temperatures measured at numerous coastal villages and using conventional methods to translate those into melt rates.
The group estimated a "net loss of about 51 cubic kilometers of ice per year from the entire ice sheet, sufficient to raise sea level by 0.13 millimeters per year." A millimeter is 4/100ths of an inch, a bit less than the thickness of a dime.
"These are very exciting and important first results," said Joe McConnell of the Desert Research Institute in Reno. (High-altitude ice formations such as those in Greenland and Antarctica qualify as deserts because of their dry air and low precipitation.) "And they show that the ice sheet is a dynamic reservoir of water that can and does influence sea level around the globe," he said.
However, he added, many factors influence the surface elevation of the ice sheet.
Taken alone, measurements of changing elevation provide little insight into the hydrological or glaciological reasons behind such change.
For example, factors such as snow accumulation over the interior of the ice sheet and rainfall on the coastal regions change significantly from one year to the next and will have a major impact on elevation.
"Others, such as increasing basal creep rates, will likely change much more slowly over decades or centuries. The bottom line is that projections of long-term trends . . . based on ice-sheet elevation measurements only five years apart are highly uncertain."
The new observations are surrounded by puzzles. The North Atlantic is currently in a colder-than-normal condition; yet as the Goddard researchers note, temperatures on the Greenland coast have been rising in the 1990s. And large regions where the laser altimeter data show significant thickening of the ice sheet "lie in areas where both ice cores and [computer] model predictions show reduced snowfall during the 1990s," Krabill's team reports.
The planned launch next year of NASA's ICESAT satellite, designed to track ice mass in the Arctic and Antarctic, may resolve some of those questions.
"The Greenland Ice Sheet is still adjusting to the climate changes reaching back to the last glacial-interglacial transition" about 13,000 years ago, veteran ice scientist Dorthe Dahl-Jensen of the University of Copenhagen writes in a commentary. "The records need to be extended to several decades before the long-term trend can be estimated."
